This invention relates to protective layers on nickel-base superalloy articles, and, more particularly, to the fabrication of such articles where the protective layer has a high content of hafnium and/or zirconium.
In an aircraft gas turbine (et) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys and can operate at temperatures of up to 1900-2100.degree. F.
Many approaches have been used to increase the operating temperature limit and operating lives of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. The articles may be formed as oriented single crystals to take advantage of superior properties observed in certain crystallographic directions. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation.
In another approach, a protective layer or a ceramic/metal thermal barrier coating (TBC) system is applied to the turbine blade or turbine vane component, which acts as a substrate. The protective layer, with no overlying ceramic layer, is useful in intermediate-temperature applications. The currently known protective layers include diffusion aluminides and NiCoCrA1Y(X) overlays, where X is typically hafnium, silicon, and/or tantalum.
A ceramic thermal barrier coating layer may be applied overlying the protective layer, to form a thermal barrier coating system. The thermal barrier coating system is useful in higher-temperature applications. The ceramic thermal barrier coating insulates the component from the exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the substrate.
Although superalloys coated with such protective layers and ceramic/metal thermal barrier coating systems do provide substantially improved performance over uncoated materials, there remains an opportunity for improvement in elevated temperature performance and environmental resistance. It has recently been discovered that incorporating hafnium, silicon, yttrium, and/or zirconium in the protective environmental coating improves its environmental resistance and adherence to the substrate. However, available techniques for applying protective layers with additions of hafnium, silicon, yttrium, and/or zirconium have not proved to be sufficiently reproducible for adoption in commercial fabrication operations. There is a need for an improved approach to preparing substrates having protective layers containing hafnium and/or zirconium. The present invention fulfills this need, and further provides related advantages.